In railroad track networks, a level traveling surface is required to ensure a quiet or smooth running of the wheels. Thus, for example, after a weld is carried out, the polished or ground traveling surface is tested with a 1-meter straightedge of extremely precise manufacture and a feeler gauge. The 1-meter straightedge is set on the traveling surface so as to be centered on the weld. In so doing, a deviation of only +0.3 mm to -0.2 mm relative to the horizontal traveling surface of the rail is allowable at the traveling surface 50 cm to the right and left of the weld. These values should also be maintained under traffic.
However, it is inevitable that deviations from the ideal traveling surface will occur occasionally after welding or in the course of traveling over the track.
In this case, it is necessary to distinguish between a positive deviation and a negative deviation.
A positive deviation of the traveling surface is observed, for example, when joint welds have a higher resistance to wear than the rail. The buckling or bulge in the traveling surface occurring over the course of time as traffic passes over can sometimes be eliminated by regrinding true to profile.
More severe problems result in the case of negative deviations of the traveling surface. Ruts which can occur in the region of the weld due to reduced resistance to wear of the weld in comparison to the rail are particularly disadvantageous. A negative deviation can also be observed when the ends of the rail are inexpertly aligned prior to welding. As the weld cools to ambient temperature, it shrinks downward. This must be compensated for by a banking or excess elevation of the ends of the rail prior to the weld.
It is particularly important to eliminate such negative deviations of the traveling surface in the case of high-speed traffic. In this case, even small ruts lead to impact stress on the joint weld which reduces its useful life. The ballast under the ties adjacent to the weld is also affected. Apart from the reduced traveling comfort due to impact stress and noise when a train passes over a rutted weld, there is also the purely economic aspect of such a negative deviation in the traveling surface.
In most cases it is impossible to eliminate a selective worn down portion in the region of a weld joint by grinding, since the rut, as such, would remain and would only be enlarged in its longitudinal dimension.